#21 Enzymes and reactions

Many chemical reactions can be speeded up by substances called catalysts. Within living organisms, these reactions (metabolic reactions) are controlled by catalysts called enzymes. Enzyme molecules are proteins.

Enzymes and reactions

Most enzyme names end in –ase, e.g. lipase, protease.

Temperature, pH and enzymes

The activity of enzymes is affected by temperature and pH.

Effect of temperature on enzymes

The optimum (best) temperature for enzyme-controlled reactions is 37⁰C (body temperature).

As the temperature increases, the rate of reaction increases. But very high temperatures denature enzymes.

The graph shows the typical change in an enzyme's activity with increasing temperature. 

The enzyme activity gradually increases with temperature up to around 37ºC, or body temperature. Then, as the temperature continues to rise, the rate of reaction falls rapidly as heat energy denatures the enzyme. Most enzymes are denatured above 50⁰C.

Effect of pH on enzymes

• The pH of a solution is how acidic or alkaline it is.
• Different enzymes work best at different pH values.
• The optimum pH for an enzyme depends on where it normally works.
• It is around neutral (pH= 7) for most enzymes but there are some exceptions.

Changes in pH also alter an enzyme’s shape and slow down its activity, but this can usually be reversed if the optimum pH is restored.

An extreme pH can denature enzymes – the active site is deformed permanently.